2.L : Navigation Systems and Radar Services

The VOR is a radio receiver, and is tuned to the frequency of the VOR station to be used. The station can be identified by a Morse code identifier, or a voice telling the name of the station. If, however, the VOR is out of service the identifier is removed and the absence of an identifier means that the station should not be used for navigation. VOR receivers can also have an alarm flag to indicate when the signal strength is too weak (because the aircraft is too low, or two far away, or is out of the line-of-sight of the station) and should not be used for navigation. Finally, even if a Morse code is detected it should be confirmed that the VOR is not broadcasting the Morse for "test", as this is sometimes also done. If in testing a VOR should not, obviously, be used for navigation.

There are two components required for VOR radio navigation. These are the ground transmitter, and the airborne receiver. The ground station is at a specific known location on the surface, and it transmits on an assigned frequency. The receiver in the aircraft can tune that frequency and has a means to display information from that signal. This instrument is traditionally a "VOR head" (though these days electronic indicators are becoming more common) which has an omni bearing selector (OBS), referred to as a "course selector", a course deviation indicator (CDI) needle, and a To/From indicator. The source selector is an azimuth dial that is rotated to select a radial (and can be used to identify the magnetic course to/from the station if desired). When the OBS is rotated the CDI moves to show the position of the radial relative to the plane.

When the OBS is rotated such that the CDI is centered, this indicates the radial (magnetic course FROM the station) or its reciprocal (magnetic course TO the station). The CDI will also move right/left if the airplane is not on the selected radial

The To/From indicator indicates whether the selected radial denotes the course TO the station, or the bearing FROM the station. If To is shown and the indicated course is flown, the aircraft will travel to the station. If From is shown and the indicated course is flown, the aircraft will fly away from the station on that magnetic course.

To track to a VOR first tune the station and verify it is the desired VOR by listening to the station identifier. Rotate the OBS to center the CDI with a "To" indication, and turn to the heading indicated on the OBS azimuth dial. This will track straight to the VOR in a no wind situation.

However, if there is a crosswind and that indicated heading is maintained the aircraft will drift off course. If the wind is from the right, the aircraft will drift left of course (and vice versa). To return to the desired radial the aircraft will have to be turned back toward the course and this correction held until the CDI re-centers. To correct for this drift and maintain the desired course the aircraft will need to crab into the wind just the right amount to counteract the crosswind. This will be a trial-and-error exercise with (hopefully) the corrections needed to maintain course being identified with a few iterations. If you happen to have a GPS the aircraft track can be shown and can confirm that the aircraft is on the desired course.

Upon arriving at, and then passing, a VOR station the "TO" indication will flip to a "FROM" indication. Generally flying away from a VOR will follow the same process. If a new course is desired at the VOR (they are often used as turn points) then a new course should be entered in the OBS.

It should be noted that if you are flying to a VOR with a "FROM" indication, or away from the VOR with a "TO" indication, that it works fine with the provision that the CDI is in a reverse sensing situation. Therefore, if the needle moves right you are actually right of the course, and vice versa if the needle moves left. This reverse sensing must be accounted for or confusion will occur.

There are a number of things to remember, or actions to take, when navigating via VOR. One is to always positively identify the station by its code or voice ID. When tracking to a station determine the inbound course and use it. Don’t succumb to the temptation to just twist the OBS to re-center the VOR. If done too much the aircraft will describe a spiral path to the station until the winds are directly in line with the course being flown, which is very sloppy flying. When flying TO a station always use a TO indication, and the reverse when flying away, thus avoiding the possible confusion of reverse sensing.

The Wide Area Augmentation System (WAAS) and Local Area Augmentation System (LAAS) were deployed to achieve a higher degree of position precision for the GPS system. This improves the position calculation enough such that the GPS can be used for precision approaches. In the worst case WAAS precision is accurate to 25 feet 95% of the time. Like GPS the WAAS includes Space, Control, and User segments.

The LAAS is very similar to WAAS, but relies more on ground stations for signal correction and improvement. However, it is considered to be less cost effective than WAAS, but is also considered to be capable of handling Category III approaches.